A wide variety of absorption processes have been proposed for removing acid gases such as, for example, carbon dioxide, hydrogen sulphide, sulphur dioxide, sulphur trioxide, carbon disulphide, hydrogen cyanide, carbonyl sulphide and others from process gas streams using absorbents comprising amines.
Such absorption processes typically involve passing the process gas stream containing one or more of the acid gases to an absorption zone wherein it is contacted with a lean solvent comprising an absorbent such as a basic solution. A product gas stream, depleted in the acid gases relative to the process gas stream, is withdrawn from the absorption zone as a product. A rich solvent stream comprising the absorbent and the absorbed acid gases is also withdrawn from the absorption zone and passed to a regeneration zone, e.g. a stripping column, wherein the absorbed acid gases are desorbed from the solvent to provide a tail gas stream comprising the acid gases and the lean solvent stream herein before described.
A common problem in such acid gas absorption processes is that heat stable salts of the base are often formed during one or both of the absorption and regeneration steps as a by-product. Heat stable salts can be formed, for example, when strong acids such as hydrochloric acid or sulphuric acid are present in the process gas.
Heat stable salts can also be formed when sulphite anions are oxidised to sulphate anions in SO2 amine recovery processes. Typical ions which form heat stable salts, i.e., heat stable anions, include, for example, sulphate anions, thiosulphate anions, polythionate anions, thiocyanate anions, acetate anions, formate anions, nitrate anions, chloride anions, oxylate ions and in addition for amines suitable for H2S and CO2 scrubbing, sulphite anions. Heat stable salts generally do not have absorption capacity for the acid gases and are not regenerable under the conditions of the process. Therefore, the level of heat stable salts needs to be controlled in order to retain an adequate degree of absorption capacity for the acid gases.
Electrodialysis has been proposed as a method for removing heat stable salts from base containing streams such as amine containing streams. In a typical electrodialysis process (see for example U.S. Pat. No. 5,910,611) caustic, e.g., sodium hydroxide, is added to the stream containing the heat stable amine salt in order to dissociate the heat stable anion from the heat stable salt and provide an amine in free base form and a simple heat stable salt, e.g., sodium sulphate. The simple salt is then separated by conventional electrodialysis wherein the charged ions permeate through anion- and cation-selective membranes. The amine, which is non-ionic, does not permeate through the membranes and is discharged from the electrodialysis zone as a product.
An alternative electrodialysis process (U.S. Pat. No. 6,517,700) achieves the removal of the heat stable anion by neutralizing the anion directly with a base in a modified electrodialysis zone. In this process, amine losses are significantly reduced. No base is added directly into the amine solution. The waste stream is composed of the neutralized anions removed from the amine stream as in the process described by U.S. Pat. No. 5,910,611.
An ion exchange process (U.S. Pat. No. 4,970,344) achieves the removal of heat stable anions by the use of suitable ion exchange resins and the use of one or both of basic and acidic solutions to regenerate the ion exchange resins.
Alternatively, thermal heat stable amine salt reclamation methods can be utilized, see for example, Kohl & Nielson “Gas Purification” fifth edition, Gulf Publishing, Houston Tex., 1997.
For all of these methods, the presence of regenerable anions in the amine stream leads to process inefficiencies, reduced throughput and increased costs. Larger sized reclamation units are needed to overcome these inefficiencies. Since the regenerable anions will behave in a similar manner to the non-regenerable anions in ionic processes, increased membrane surface area or ion exchange resin volume will be required. Increased quantities of neutralization chemicals and electric power and/or regeneration chemicals will also be required. For thermal processes increased neutralization chemicals will be required.
For all of these methods, the quantity of waste generated increases with the quantity and concentration of non-regenerable anions in the solution being treated. Typically higher waste volumes will result in proportionally greater amine losses.
Primary and secondary amine amines can form carbamates when reacted with CO2 as described in reaction 1 below for the case of a secondary amine.R2NH+CO2→R2NHCOO−+H+
The resultant carbamate is ionic and will be removed with other heat stable and non-heat stable anions in ionic (electrodialysis and ion exchange) reclamation processes, resulting in an increase in amine losses.
Thus ensuring that the amine being treated is as lean as possible especially with respect to its lean CO2 loading is imperative in order to reduce amine losses and maximize removal efficiency and capacity.
In certain scrubbing applications there is little need to achieve low acid gas concentrations in the treated gas. An example of this is carbon capture from flue gases, where 90% overall acid gas recovery is acceptable. Treated gas CO2 concentrations may thus be greater than 1%. Such high concentration treated gas does not necessitate a high degree of leanness in the lean amine feed to the absorption tower.